Ultrasonic sensor system for horizontally polarised transversal waves

ABSTRACT

The invention relates to an ultrasonic sensor system, especially for controlling a resistance welding process. Said system comprises at least one receiver which is used to detect the ultrasonic signals from the area to be examined. At least two piezoelectric sensors ( 31, 32 ) are used as a receiver and are arranged in such a way that their polarisation direction vectors indicate various directions, said vectors being projected in a plane perpendicular in relation to the propagation direction of an ultrasonic wave to be detected.

BACKGROUND INFORMATION

[0001] The present invention relates to an ultrasonic sensor system forcontrolling a resistance spot welding process according to the generalclass of the independent claim.

[0002] The essence of the method described in European PatentApplication EP-A-653 061 is to investigate the intended weld region byultrasonic transmission during the welding operation using shear andtransversal waves by situating an ultrasonic transmitter and anultrasonic receiver for shear waves on each of the external electrodeadapters of the two diametrically opposed welding electrodes. Startingat the ultrasonic transmitter on one welding electrode, the ultrasonicsignal passes through the weld material—two or more sheets to bewelded—and the other welding electrode until it reaches the ultrasonicreceiver. Said ultrasonic receiver converts said ultrasonic signal to ameasurable electrical signal U, the temporal course of which can bedepicted using the equation U=U_(O)·sin ωt. In this equation, ω is theangular frequency of the ultrasonic wave, and t is the time. Thethrough-transmission signal is detected online, and its amplitude U_(O)is used as the control variable for amplitude and the shape of thewelding current curve over time. The transversal wave is selectedbecause the influence of fluid formation in the weld nugget on thedampening of a through-transmitted wave is very strong with this type ofwave. The amplitude U_(O) of the transversal wave—which changes markedlyand in characteristic fashion over the course of the weldingprocess—permits a reliable determination of the formation and size ofthe weld nugget and can therefore be used as a manipulated variable fora control process.

[0003] The basic feasibility of the method and the reliability of theexamination findings are crucially dependent on the ultrasonic sensorsused, their location relative to the welding electrodes, and the soundpropagation inside the welding electrodes. In the realization accordingto EP-A-653 061, an arrangement of ultrasonic sensors is selected inwhich the ultrasonic transmitter and ultrasonic receiver are mounted onthe external electrode adapters or on the electrode holders, which arenot shown in the drawing. Shear waves, transversal waves, or torsionalwaves having a frequency of less than 1 MHz are generated. It is statedthat it is particularly advantageous to generate horizontally polarizedtransversal waves, since they have a low tendency to undergo undesiredmode changes when reflections occur inside the sound-directingelectroder holder.

[0004] Transversal or shear waves propagate only in solid bodies, andnot in fluids. In these types of waves, the particles or atoms oscillateperpendicular to the propagation direction of the wave. The direction ofoscillation of the particles or atoms is also referred to as thepolarization direction or, within an imagined coordinate system, as thepolarization vector.

[0005] Transversal waves that propagate in the longitudinal directioninside a longitudinally-extending, laterally-limited solid body, e.g., aplate or a hollow cylinder, are said to be “horizontally polarized” whenthe polarization vector of the sound wave, i.e., the direction ofoscillation of the particles or atoms, is parallel to one of the laterallimiting surfaces. If, for example, a transversal wave is coupled intopart of the end surface of a hollow cylinder, which said transversalwave propagates in the axial direction of the cylinder, it ishorizontally polarized if its polarization vector points in a tangentialdirection of the cylinder.

[0006] The ultrasonic transmitters and receivers are “shear wave testheads”. They contain flat and, usually, round piezoelectric plateshaving a diameter ranging from a few mm to a few cm, and that execute ashearing motion when excited with electric voltage or, conversely, whenthey receive, they react to a received shear wave with a receptionvoltage. Since, when a shear wave test head of this type is mounteddirectly on the external electrode adapter, the main emission directionof the sound would not be directed in the direction of the weldmaterial, but rather at the center of the electrode, wedge-shapedattachments are preferably used, that are installed between the testheads and the welding electrodes and permit the main emission directionof the test head to be oriented toward the weld material at an anglethat is markedly less than 90°, e.g., approximately 45°. This is theonly way to bundle an adequate portion of the sound energy toward thewelding spot with this sensor arrangement.

[0007] German Patent Application DE-A-199 37 479, which was published ata later date, describes an ultrasonic sensor system that is improved inthis regard. With said ultrasonic sensor system, the piezoelectric shearwave plate or the complete shear wave test head is installed in a recessinside the electrode adapter for transmitting and/or receiving. In fact,said piezoelectric shear wave plate or the complete shear wave test headis installed in such a manner that the piezoelectric plate is orientednearly perpendicular to the electrode adapter, and the main emissiondirection of the transmitter and the main reception direction of thereceiver are therefore parallel to the electrode adapter and aredirected exactly at each other. This allows such a level of ultrasonicintensity to be produced in the welding spot and, during reception, itallows a received signal to be generated that is so great that anadequate wanted-to-unwanted signal ratio exists with regard for thefurther evaluation for controlling the welding process. Rectangularpiezoelectric shear wave plates are used in this case. Basicallyspeaking, however, they can have another geometric form (e.g., round,semicircular, or rhombic) as well.

[0008] Very generally speaking, if material areas to be examined areinvestigated by ultrasonic transmission using a separate ultrasonicshear wave transmitter and a separate ultrasonic shear wave receiver,there is always the difficulty that the transmitter and receiver must bedirected at each other exactly with regard for the polarizationdirection of the shear wave produced. To provide the user with a roughorientation, the particular polarization directions are therefore alwaysmarked on the housing when shear wave test heads are used. In atransmitter-receiver arrangement, the polarization directions of thetransmitter and receiver must match, because the two ultrasonic shearwave test heads behave, in terms of the amplitude of the electricalreceived signal, like two optical polarization filters in terms of thepassage of light: if the two shear wave test heads are in exact parallelalignment and the maximum reception voltage is U_(O), the receptionvoltage is U(α), depending on the angle α at which the two polarizationdirections are rotated relative to each other:

U(α)=U _(O)·cos(α)·sin(ωt)

[0009] (ω=angular frequency, t=time)

[0010] When α=90°, the amplitude U_(O)·cos (α) of the reception voltageU(α) is theoretically zero. Due to diffraction and refraction phenomena,and the natural sound field characteristics of a piezoelectric disk,however, a finite value is still usually measured for U(α) when α=90°.Said value is so small, however, (1 to 10% of U_(O)), that the receivedsignal can no longer be reliably evaluated.

[0011] These facts also affect the sensor systems described hereinabovefor monitoring a resistance spot welding process, in particular: thepolarization directions of the ultrasonic shear wave transmitter andreceiver installed on the electrode adapters or integrated in theelectrode adapters must be directed toward each other and mounted insuch a manner that their polarization directions are parallel to eachother. If not, the through-transmission amplitude is too low. When theshear wave sensors are mounted on the electrode adapters, or theelectrode adapters are installed in the electrode holders when thesensors are integrated in the adapters, an adjustment step must becarried out. To do this, the sensors and/or the electrode adapters withthe preinstalled sensors are turned, in a first rough step, until onecan see that the markings of the polarization directions of thetransmitter and receiver are parallel with each other. A fine adjustmentis then carried out, again by turning the sensors or the electrodeadapters. To do this, the reception voltage is observed and brought to amaximum value. This procedure is complicated, time-intensive, andfraught with error if it is not carried out with the proper level ofcare. It must be carried out by trained technicians, because theultrasonic signal must be observed and interpreted as well, for controlpurposes. If the sensors or electrode adapters are worn, they cannot besimply replaced by untrained personnel. When the sensors are replaced,one must also put up with an undesirably long period of downtime of thewelding machine because of the adjustment that must be carried out.

[0012] The object of the present invention is to provide a sensor systemfor shear waves that functions without having to set up the transmitterand receiver as described hereinabove, enabling the sensors to bereplaced easily during initial installation or when they are worn, whenthey are used for controlling a resistance spot welding process. Thisobject is attained by the features of the independent claim.

ADVANTAGES OF THE INVENTION

[0013] The ultrasonic sensor system, in particular for controlling aresistance spot welding process, has at least one receiver that detectsthe ultrasonic signals from the area to be examined, whereby at leasttwo piezoelectric sensors are used as a receiver that are arranged insuch a way that their polarization direction vectors indicate variousdirections, said vectors being projected in a plane perpendicular to thepropagation direction of an ultrasonic wave to be detected. This insuresthat at least one of the piezoelectric sensors detects a signal—that isdifferent from zero—independently of the polarization direction of thewave to be detected. In particular, it is independent of how thereceiver is positioned relative to the transmitter. As a result, complexadjustment procedures can be eliminated. The downtimes of resistancespot welding systems can therefore be greatly reduced.

[0014] In an advantageous further development, it is provided that theoutput variables of the at least two sensors be coupled in a signalprocessing unit accordingly in order to detect a measure of theamplitude of the ultrasonic wave. This coupling increases thesensitivity of the system. Using the types of coupling named in thefurther dependent claims, it can be insured that the output signal doesnot fall below a certain minimum level. This increases the reliabilityof the evaluation and, therefore, the quality of the controlling of theresistance spot welding process.

[0015] In an advantageous further development it is provided that thepiezoelectric plates have a stacked configuration. This results in theabsence of lateral misalignment, in particular, so that the sound fieldis absorbed by both piezoelectric plates at the same point. This makesthe arrangement particularly suited for use with any spaciallyinhomogeneous ultrasonic wave field. The signal processing unit can makeappropriate corrections to easily compensate for the phase displacementthat occurs in terms of the sound propagation time.

[0016] Further advantageous developments result from the furtherdependent claims and the description.

[0017] The present invention provides that, rather than using a singlepiezoelectric shear wave receiver, a plurality of identical shear wavereceivers are used, the polarization directions of which are located ina common plane, but that have various directions within the plane, sothat a shear wave that is propagating at a right angle to this planealways delivers a received signal that is different from zero to atleast one of the receivers, independently of its polarization directionin this plane, and that the reception voltages of the individual shearwave receivers are transmitted to an electronic circuit device thatgenerates an output signal by suitably coupling the individual receptionvoltages, which said output signal is different from zero and isproportional to the amplitude of the shear wave to be received given anyposition of the polarization direction.

[0018] In terms the application for controlling a resistance spotwelding process, the present invention is based, in particular, on theknowledge that a low-frequency (<1 MHz) shear wave that is introducedinto the welding electrode—which is cylindrical and hollow inside inorder to accommodate the cooling water—propagates more or lesshomogeneously through the entire cross section of the welding electrodeon its way to the receiver on the other welding electrode. This is dueto the fact that, with typical propagation speeds of 3000 m/s, thewavelength of the shear wave in the cylindrical shaft of the weldingelectrode ranges from a few millimeters to a few centimeters. Weldingelectrodes typically have an outer diameter of 15-30 mm, and their wallsare typically 4-8 mm thick. The magnitude of the cross section of theelectrode adapter is therefore equal to or smaller than that of thewavelength. The cross section of the welding electrode itself is alreadysuch a small aperture opening for the propagating ultrasonic wave that anearly undirectional propagation of sound takes place, and the soundwave fills the entire cross section of the electrode adapter after justa short path of travel.

DRAWING

[0019] Exemplary embodiments are shown in the drawing and are describedin greater detail hereinbelow.

[0020]FIG. 1 shows the fundamental mode of operation of the sensorsystem using, as an example, two piezoelectric disks that are positionedat a 90-degree angle relative to each other,

[0021]FIG. 2 shows the sensor system in combination with a circuitdevice,

[0022]FIG. 3 shows a further sensor system outside of the weldingelectrode,

[0023]FIG. 4 shows a sensor system integrated in the welding electrode,and

[0024]FIG. 5 shows a further sensor system having a stackedconfiguration.

[0025]FIG. 1 shows the fundamental mode of operation of the sensorsystem, according to the example, using the simplest example of twopiezoelectric disks, 31 and 32, that are positioned at a 90-degree angle(α1-α2) relative to each other. The two piezoelectric disks 31 and 32,that are designed as shear wave oscillators, are positioned, asreceivers, in a shear wave field that may be homogeneous within the areaenclosed by line 33. The propagation direction of the shear wave extendsperpendicular to the plane of the paper. P1 and P2 are the polarizationvectors (and polarization directions) of the two piezoelectric disks 31and 32. P3 is the polarization vector of the shear wave that is passingthrough the plane of the paper. α1 is the angle that exists between thepolarization vector P3 of the shear wave and the polarization vector P1of the first piezoelectric disk 31. α2 is the angle that exists betweenthe polarization direction P3 of the shear wave and the polarizationvector of the second piezoelectric disk 32. This means that thereception voltages U1 and U2 of the shear wave sensors 31 and 32 are:

U 1=U _(O)·cos(α1)·sin(ωt)=A 1·sin(ωt)

U 2=U _(O) ·cos(α2)sin(ωt)=U _(O)·cos(α1−90°)·sin(ωt)=U_(O)·sin(α1)·sin(ωt)=A 2·sin(ωt)

[0026] (ω=angular frequency of the ultrasonic wave, t=time)

[0027] Accordingly, of the amplitudes A1=U_(O)·cos (α1) and A2=U_(O)·sin (α1) of the received signals U1 and U2 of the two shear wavesensors or piezoelectric disks (31, 32), at least one of them is alwaysdifferent from zero. According to FIG. 2, the received signals U1 and U2of the two shear wave sensors or piezoelectric disks 41, 42 are nowforwarded to a circuit device 44 that can couple the individual receivedvoltages in diverse fashions in such a manner that a single outputsignal Ug (e.g., Ug=Ag·sin (ωt)) results, the amplitude Ag of which ismore or less independent of the polarization direction of the shear waveto be detected.

[0028] The same observations also apply, in a similar sense, when thepiezoelectric shear wave plates used for reception in FIG. 1 arepositioned so that they are tilted relative to the plane of the paperand form an angle γ with the impinging wave front, which said angle γ isdifferent from zero. In this case, the same observations apply for thevectors of the polarization directions of the shear wave test heads orthe shear wave piezoelectric plates, which said vectors are projectedinto the plane that is perpendicular to the propagation direction of theshear wave to be detected.

[0029] The following examples of coupling the received signals in thesense of the present invention are easy to carry out in terms ofcircuitry using analog, integrated circuits (IC's), or by digitizing theultrasonic signals, followed by arithmetic operations:

[0030] a) First of all, signals U1 and U2 of the shear wave sensors orpiezoelectric plates are squared and then added.

Ug=U 1 ² +U 2 ² =Uo ²·cos²(α1)·sin²(ωt)+Uo ²·sin²(α1)·sin²(ωt)=Uo²·sin²(ωt)

[0031] Since cos²(α1)+sin²(α1) is always equal to 1, the resultantamplitude of the received signal is always Uo², completely independentof angle α1, the squared reception voltage amplitude of an individualshear wave receiver with its polarization direction oriented parallel tothe polarization direction of the shear wave to be detected.

[0032] b) Based on Case a), the result is extracted further:

Ug={square root}{square root over ((U 1 ² +U 2 ²))}=Uo·{squareroot}{square root over(cos²(α1)·sin²(ωt)+sin²(α1)·sin²(ωt)))}=Uo·|sin(ωt)|

[0033] In this case, the received signal Ug—independent of angleα1—always corresponds exactly to the received signal of an individualshear wave receiver with its polarization direction oriented in parallelwith the polarization direction of the shear wave to be detected.

[0034] c) The absolute values (amounts) of the two reception voltagesare determined and added:

Ug=|U 1|+|U 2|=Uo·|sin(ωt)|·(|cos(α1)|+|sin(α1)|)

[0035] This result is already fully sufficient as well when thepolarization directions of the transmit and receive sensors are alignedwith each other in fixed fashion, as is the case when they are used inresistance spot welding systems after the sensors are installed in thewelding tongs: In this case, the amplitude of Ug would beU_(O)·(|cos(α1)|+|sin(α1)|), and, depending on the installation of thesensors and the angle (α1), it would always be between 1 and {squareroot}{square root over (2)}, but never 0. Independent of angle α1, anadequate reception voltage would therefore always be available withoutthe polarization directions from the transmitter and receiver having tobe directed toward each other.

[0036] d) The absolute values (amounts) of U1 and U2 can also bedetermined, first of all, then both amounts can be compared, and thegreater of the two can then be used as the output signal Ug. In thiscase, the amplitude of Ug would always be between 1 and ({squareroot}{square root over (2)})/2:

Ug=Max(|U 1|, |U 2|)=Uo·sin(ωt)|·Max(|cos(α1)|,|sin(α1)|

[0037] The arrangement of two or more receiving sensors, the receptionvoltages of which are processed further in appropriate fashion accordingto this invention, as described hereinabove, for example, can berealized in the most diverse fashion. If a sensor system according toEP-A-653 061 having shear wave test heads mounted on the lateralelectrode adapter for controlling a resistance spot welding process isselected, then a second, identical shear wave test head 52 can bemounted on the electrode adapter 5.1 on the receiver side next to thefirst shear wave test head 51, as shown in FIG. 3. The installation siteis so selected that the position is identical with regard forlongitudinal axis 55 of the electrode adapter, and an angulardisplacement of only 90° (FIG. 3), for instance, exists in the planeperpendicular to longitudinal axis 55 of the welding electrodes.

[0038] With a sensor system according to DE-A-1 99 37 479 having sensorsintegrated in the electrode adapters, two shear wave sensors having apolarization direction offset by 90° can be installed on the receivingside within a cross section 66 of electrode adapter 6.1 that lies in aplane 66 perpendicular to central axis 65 of the electrode adapter,e.g., by positioning two otherwise identical piezoelectric shear waveplates 61 and 62 offset by exactly 90° (FIG. 4).

[0039] Instead of positioning two or more shear wave receiving sensorsside-by-side in the ultrasonic shear wave field, it is alsofundamentally possible to position shear wave sensors in the same placein the sound field, which said shear wave sensors have polarizationdirections that are offset according to the invention. FIG. 5 shows thisusing a simple example, in which only two receiving sensors are usedthat have a polarization direction offset by 90°: In this case, the factthat piezoelectric transducers can also be configured and producedhaving a stacked design is utilized. Accordingly, in FIG. 5, twoidentical shear wave piezoelectric plates 71, 72 having polarizationdirections P71, P72 offset by 90° are stacked one on top of the other,in alignment with each other. The two shear wave piezoelectric plates71, 72 are joined with each other in acoustically conductive fashion,e.g., by surface bonding or soldering. Electrical leads 78, 79 are soinstalled on the surfaces of the piezoelectric disks that the receptionvoltages U71, U72 of the two piezoelectric plates 71, 72 can be pickedoff separately at said electrical leads. Further details of thetransducer construction, such as protective layers or dampening bodies,are designed in accordance with the related art. They are left out ofFIG. 5, because they are not a subject of the invention, and they arenot required any further to explain the mode of operation of the presentinvention.

[0040] It is true that ultrasonic test heads having a stackedconfiguration in accordance with FIG. 5 are basically more complex tomanufacture in practice than conventional test heads having just one plyof piezoelectric elements. The present case has the advantage, however,that no lateral displacement whatsoever exists between the individualpiezoelectric plates or sensors. Instead, the shear wave sound field isabsorbed by both of the piezoelectric plates at the same point, exceptfor a displacement in the sound propagation direction. This embodimentof the invention can therefore be used with every spaciallyinhomogeneous shear wave field as well. With reception voltages U71 andU72, the displacement in the sound propagation direction is realizedonly as a slight phase displacement in terms of sound propagation time,which said phase displacement can be compensated for or neglectedelectronically or arithmetically in the further processing of receptionvoltages, according to the invention, in a signal processing unit.

[0041] The invention is not limited to the use of a horizontallypolarized transversal wave that is always bound to a lateral transfermedium (rod, plate, electrode adapter). The invention functions with anytransversal wave, independently of whether it propagates in a limited orunlimited medium.

What is claimed is:
 1. An ultrasonic sensor system, in particular fortransversal and/or shear waves, having at least one receiver whichdetects the ultrasonic signals from the area to be examined, wherein atleast two piezoelectric sensors (31, 32) are used as a receiver and arearranged in such a way that their polarization direction vectors (P1,P2) indicate various directions, said vectors being projected in a planeperpendicular to the propagation direction of an ultrasonic wave to bedetected.
 2. The device as recited in claim 1, wherein the sensors (31,32) are arranged in such a way that the polarization direction vectors(P1, P2) are preferrably offset by
 900. 3. The device as recited in oneof the preceding claims, wherein the output variables from the at leasttwo sensors (31, 32) are transmitted to a signal processing unit (44)that delivers an output signal depending on the output variables of thesensors (31, 32), which said output signal is a measure of the amplitudeof the ultrasonic wave to be detected.
 4. The device as recited in oneof the preceding claims, wherein the sensors (31, 32) are preferablylocated side-by-side in a plane.
 5. The device as recited in one of thepreceding claims, wherein the sensors (31, 32) are composed ofpiezoelectric plates (71, 72) that are stacked one on top of the other,in alignment with each other.
 6. The device as recited in one of thepreceding claims, wherein the sensors (31, 32) are integrated in theshaft of an electrode (6.1) for resistance welding.
 7. The device asrecited in one of the preceding claims, wherein the output variablesfrom the sensors (31, 32) are coupled by adding the squared individualsignals and/or determining the root of the addend.
 8. The device asrecited in one of the preceding claims, wherein the output variablesfrom the sensors (31, 32) are coupled by adding the amounts of theindividual signals.
 9. The device as recited in one of the precedingclaims, wherein the output variables from the sensors (31, 32) arecoupled by forwarding only the greater of the individual signals forfurther processing.
 10. The device as recited in one of the precedingclaims, characterized by its use for controlling or monitoring aresistance welding process.